1. Field of the Art
Embodiments of the present invention generally relate to wireless power transfer to surgically implanted prostheses, in particular, to a three-coil power transfer system with one of the coils being a fully intraocular coil.
2. Description of the Related Art
Age-related macular degeneration (AMD) and retinitis pigmentosa (RP) are two most common outer-retina degenerative diseases of the human eye. There is promise in the use of retinal prostheses in order to allow people afflicted with the diseases to see. Retinal prostheses, which bypass the defective outer-retina photoreceptors and electrically stimulate the inner-retina neurons directly, have allowed some blind people with AMD and RP to perceive light.
It is recognized that these early prostheses only involve a very small number of stimulating electrodes on the neurons. To realize facial recognition or large-sized letter reading, next-generation retinal prosthetic devices may use 1024 or more stimulating electrodes.
Unfortunately, the high-resolution sensors and processors have relatively high power consumption, for example, greater than 100 milliwatts (mW). Current battery technology limits their usefulness for such implants, so power is preferably drawn from outside the body. Cables tend to be unwieldy for connecting a patient's eye to an external power source, so wireless power transfer is preferred.
Electromagnetic inductive coupling between two coils has been widely studied and optimized for wirelessly powering retinal prosthetic devices. However, because of the extremely demanding physical constraints in and around the eye, the physical placement of the implanted receiver coil remains a matter of ongoing debate. There are tradeoffs between the power-transfer capability, surgical risk and long-term implantation. While there are fewer constraints on a transmitting coil, which is outside the body, it cannot be too powerful lest it heat the receiver coil too much or subject the patient to unacceptable levels of electromagnetic fields.
Generally, the inductive link efficiency between two coils is proportional to the square of the coupling coefficient (k) and the respective quality factors (Qs) of coupled coils. Wireless power transfer naturally involves no magnetic transformer core around which a primary winding and a secondary winding are wound as in a conventional electrical transformer. Instead, wireless inductively-linked coils are coupled through the air (or other intervening media).
In the prior art, to compensate for the low-efficiency of this air-cored coupling and satisfy safety limitations, such as heat dissipation, electromagnetic field exposure, etc., the receiver coils are placed extraocular and connected to the electrodes sitting intraocular through a cable that penetrates the eyeball. To penetrate the eyeball into the inside, one typically penetrates the eye's sclera and choroid. This trans-sclera, trans-choroid cable potentially causes infection and hypotony in the long-term implantation.
Fully-intraocular retinal implants have been attempted that place the receiver coil inside the lens capsule after removing the natural lens. However, with a 25 millimeter (mm) (1-inch) separation between the transmitter and receiver coils, this 2-coil configuration suffers low efficiency (e.g., 7%) from the limited Q of the receiver coil and the small coupling coefficient k between the coupled coils.
There exists a need in the art for more efficient wireless electrical power transfer methods for retinal implants.